Installation of a sub panel

I would like to modernize / upgrade the electical supply panel in my son-in-laws garage which is located about 140 feet from the house. Reading through some of the posts on this site, it seems a sub panel (either 60a or 100a) is the answer as he would like to be able to use a 50a welder. How would a 100a sub panel be connected into the main 200a panel in the house. A 100a breaker? (not sure such a thing is available) Connected directly to the bus on the main breaker and then go through a fused disconnect before leaving the house?

A 100A subpanel would be supplied via a 100Amp double-pole breaker from the main panel. The main panel is not in the garage, right? What wires are running from the house to the garage now?

You will likely need to run 4 conductors through conduit buried 18" from the house to the garage, as well as drive two ground rods into the ground at the garage.

Correct, the main (200a panel is in the basement of the house. The garage is currently supplied via a 40a breaker in the main panel that goes to a ratty old fuse panel in the garage. It looks like 60a fuse panel but it has seen better days. The cable is directly buried between the garage and house. I am not sure what size it is but I expect it will need to be replaced. (back breaking but doable!). Suggestions on what to repalce it with?

I'd run PVC conduit with THWN wire. I think you need (3) #3 THWN plus a ground. The ground can be a smaller conductor, I am not sure how much smaller (hence the thread I just started).

Most code requires 18" deep to the top of the conduit.

You also need ground rods for a subpanel at a detached structure.

Neutral bar and ground bar must NOT be connected in any way in a subpanel. You can test this with a continuity tester before even supplying any power to the subpanel. There should not be continuity. You may have to remove a bonding strap if you buy a main breaker panel.

As long as you feed the panel via a 100A breaker in the main panel, the shutoff in the garage can be 100A or larger.